PHP preg_replace anything that isn't string with danish characters

Question

In PHP I am trying to replace anything that isn't a character (a-z and æ, ø and å) using this function:

$word = preg_replace('/[^a-z]/i', '', $word);

However, this will also replace the danish characters of æ, ø and å.

Testing different online tools using:

/[^a-zæøå]/i'

Yields the result I'm after, but using this in my PHP:

$word = preg_replace('/[^a-z]/i', '', $word);

Does not yield my expected result. I hope someone may be able to explain why that is, and what I should change to get the expected behavior.

Test data:

Cykeltøj
Shouldn't
abæøå

Expected output:

Cykeltøj
Shouldnt
abæøå

Actual output in the first function:

Cykeltj
Shouldnt
ab

Actual output in the second function

*nothing*
*nothing*
*nothing*